The use of magnetic fluid and ferrofluid seals for sealing emissions through rotary shafts is well known. Magnetic fluid or ferrofluid magnetic seals have been used as exclusion seals, where pressure differences on either side of the ferrofluid seal are insignificant, or as multiple stage seals, where there is a difference in pressure environment between the different sides of the seal.
Generally, multiple stage pole pieces are used in ferrofluid or magnetic seals to typically provide a seal about a rotating shaft between high and low pressure environments, for example, or to exclude contaminants from reaching a sensitive part of a device or an environment protected by the seal.
The typical design of a ferrofluid exclusion seal consists of a permanent magnet and two magnetic permeable pole pieces, with the magnetic-flux circuit completed through the magnetically permeable shaft, to be sealed.
Conventionally, ferrofluid seal is comprised of a precisely dimensioned ring-shaped permanent magnet, ring-shaped pole pieces and ferrofluid. The magnet is positioned around the shaft or bearing and the pole pieces are mechanically attached to the magnet faces and extend close to the shaft. The magnets do not touch the shaft and the gaps between are filled with ferrofluid.
The pole pieces are conventionally formed from a magnetically permeable metal and the ferrofluid is comprised of a suspension of magnetically permeable particles in a fluid carrier so that the pole pieces, the magnet, the ferrofluid and the shaft or bearing form a closed-loop magnetic circuit. The magnetic flux generated by the magnet passes through the ferrofluid and holds it in the gaps between the pole pieces and the shaft to form the seal. The construction and operation of such seals are described in detail in U.S. Pat. Nos. 4,407,508; 4,630,943; 4,628,384; and 4,357,022 and will not be discussed further herein.
U.S. Pat. No. 4,927,164 to Raj et al. describes a ferrofluid seal which comprises a magnet and at least one pole piece. The pole piece is fabricated from a ferromagnetic material which directly bonds to the magnet face, thereby eliminating the need for additional adhesive for enhanced seal integrity.
For example, U.S. Pat. No. 4,607,500 to Raj et al. describes a system in which a single-pole-piece ferrofluid exclusion seal is employed.
The single pole-piece ferrofluid seal apparatus of the invention consists of a single magnetically permeable pole piece, with the single pole piece at one end in a magnetic-flux relationship with a permanent magnet, and with the other end extending into a close, non-contacting relationship with the surface of the shaft element to be sealed. The small radial gap which is defined therebetween is filled with ferrofluid to provide the ferrofluid seal. The single pole piece is secured to the permanent magnet by bonding means.
The single pole-piece ferrofluid seal is unsuitable for providing a seal between high and low pressure environments. Furthermore, the single pole piece has a tendency to separate from the magnet.
U.S. Pat. No. 4,445,696 to Raj et al. describes a ferrofluid seal for use in high vacuum applications where one pole piece is separated from the shaft by ferrofluid seal, the other pole piece adjacent to the vacuum side filled with an epoxy non-magnetic type resin to reduce the air gap. Higher pressures "push" the ferrofluid seal away from the pole piece and the seal is unsuitable for pressure differences exceeding 6-7 psi.
Commonly, prior art ferrofluid and magnetic fluid seals used to seal emissions around a shaft, for example, are attached to the end flange of the mechanical seal, away from the bearings supporting the shaft. Because of the length of the shaft and of the cumulative effect of all vertical and horizontal dimensional imperfections, the resulting shaft eccentricity affects adversely the performance of the seal. The performance of the magnetic fluid seal depends on the ability to maintain a constant gap between the rotating shaft and the magnetic pole pieces. In the presence of distortions, this gap must be at least equal to the sum of the gap required and the magnitude of the shaft distortions. Consequently, the farther the seal is located from the shaft bearing support, the larger must be the gap between the shaft and the magnetic pole pieces. This in turn, affects the maximum pressure which the magnetic fluid seal is able to withstand. Prior art solutions, such as U.S. Pat. No. 4,445,696, are restricted to a maximum permissible pressure of 6 psi per module width. Since the minimum required pressure in a typical petrochemical application is at least 20-25 psi, four modules would have to be used making it unsuitable for retrofit applications or for pumps not having sufficient space for more than one unit.
Also, this additional seal length would add further to the cumulative shaft distortions requiring a larger still gap size for the magnetic fluid. Since there is a cubic relationship between the gap size and the seal size, that is doubling the gap size requires an eight-fold increase in the seal volume, space limitation on existing pumps becomes a decisive factor.
Furthermore, the use of multiple modules significantly increases the costs of the seals.